1. Field of the Invention
This invention relates generally to wireless links and more particularly to an antenna structure for communicating between fixed-position transmitter and/or receiver nodes.
2. Description of the Related Art
Wireless services are conventionally provided between nodes on a network using two traditional types of access configurations. In a conventional point-to-point (PTP) access configuration, a fixed-position transmitter node makes a wireless link to a single fixed-position receiver node. Since the position of the receiver node is generally known a priori and there is only one receiver node, the antenna used in the transmitter node can be a narrow-beam antenna in order to concentrate RF power in the direction of the receiver node.
In a conventional point to multi-point (PMP) access configuration, a fixed-position transmitter node makes a wireless link to several receiver nodes. In a conventional PMP access configuration, the antenna used in the transmitter node may be a broad-beam antenna (i.e., an antenna having a fairly wide antenna gain pattern) so that the RF signal can reach all of the receiver nodes in the desired locations (i.e., within the coverage volume of the antenna). An example of a PMP system is a cellular network, where a single base station communicates with many cell phones simultaneously. Another example is satellite communications where a single satellite communicates to multiple earth stations. In both PTP and PMP configurations, the wireless links can be simplex (i.e., one way) or duplex (i.e., two way).
Network coverage implemented by the PMP approach can have several advantages compared to the same network coverage implemented using solely PTP links. First, PMP links generally require less hardware (and therefore less capital) since a single transmitter node (e.g., a hub node) can broadcast its RF signal to many receiver nodes. In the equivalent PTP system, one transmitter node would be required for each receiver node. Second, PMP systems can be configured to allow operators flexibility to quickly add subscribers onto the network without having to install additional transmitters. For example, if a single hub is transmitting to subscribers within a certain coverage volume, then little if any additional equipment is required at the hub in order to add another subscriber within the coverage volume. In contrast, if solely PTP links were used, additional equipment would have to be installed at the hub in order to set up an additional PTP link between the hub and the new subscriber. Further, PMP solutions generally will reduce the number of antennas at any given location and the space required to house the transmitter node, since a single broad-beam antenna will generally take the place of several narrow-beam antennas. This can significantly reduce the costs (e.g., rent or purchase price) for the associated space and can reduce the costs for installation. Further, by utilizing a single broad-beam antenna instead of several narrow-beam antennas, there is decreased tower loading. Thus, the need for structural upgrades to the towers is decreased. Using fewer antennas and reducing support structures for the towers can improve building and site aesthetics at the location where the transmitter node is installed. This can reduce zoning concerns by minimizing the impact to the skylines.
However, PMP systems generally do not provide the same range as PTP systems for a given level of RF power. As the beamwidth of the transmitting antenna increases (as is generally the case for PMP nodes compared to PTP nodes), the antenna gain decreases as does the range of the transmitter node. This is because PMP nodes generally use antennas with wider beamwidths. Hence, the radiated RF power is distributed over a larger area (or volume), to provide the maximum coverage and is not as concentrated at any one point, as is typically the case with narrow-beam antennas used in a PTP configuration.
If the coverage volume of the PMP node is sparsely populated with receiver nodes, then much of the RF power may be directed to locations where there is no receiver and is therefore wasted. Wasted RF propagation can also be viewed as interference, which reduces spectrum efficiency and capacity. For example, a PMP node may use an omnidirectional antenna (i.e., an antenna with a 360° beamwidth) because the receivers may not be concentrated in any general area. However, there may be an obstruction, such as a building, that obstructs a significant part of the area around the omnidirectional antenna. In this case, the RF power radiated in the direction of the obstruction is wasted since there are no receiver nodes located in that direction.
A similar situation exists for receiver nodes.
Therefore, there is a need for transmitter and/or receiver nodes that overcome some or all of the disadvantages described above.